linux-kernel - Q: sys_perf_event_open() && PF

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Date:	Mon, 8 Nov 2010 15:57:25 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Q: sys_perf_event_open() && PF_EXITING

I am puzzled by PF_EXITING check in find_lively_task_by_vpid().

How can it help? The task can call do_exit() right after the check.

And why do we need it? The comment only says "Can't attach events to
a dying task". Maybe it tries protect sys_perf_event_open() against
perf_event_exit_task_context(), but it can't.

c93f7669 "perf_counter: Fix race in attaching counters to tasks and
exiting" says:

    There is also a race between perf_counter_exit_task and
    find_get_context; this solves the race by moving the get_ctx that
    was in perf_counter_alloc into the locked region in find_get_context,
    so that once find_get_context has got the context for a task, it
    won't get freed even if the task calls perf_counter_exit_task.

OK, the code was changed since that commit, but afaics "it won't be
freed" is still true.

However,

    It
    doesn't matter if new top-level (non-inherited) counters get attached
    to the context after perf_counter_exit_task has detached the context
    from the task.  They will just stay there and never get scheduled in
    until the counters' fds get closed, and then perf_release will remove
    them from the context and eventually free the context.

This looks wrong. perf_release() does free_event()->put_ctx(), this pairs
get_ctx() after alloc_perf_context().

But __perf_event_init_context() sets ctx->refcount = 1, and I guess this
reference should be dropped by ctx->task ? If yes, then it is not OK to
attach the event after sys_perf_event_open().

No?

Hmm. jump_label_inc/dec looks obviously racy too. Say, free_event() races
with perf_event_alloc(). There is a window between atomic_xxx() and
jump_label_update(), afaics it is possible to call jump_label_disable()
when perf_task_events/perf_swevent_enabled != 0.

Oleg.

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